The present invention pertains to stabilizing at high temperatures urethane elastomers prepared with organo-metallic catalysts.
Polyurethane elastomers have acquired an important place in the present-day technology of synthetic resins, and one notable use of this elastomer is as a tire-filling material to convert a pneumatic tire into a solid, flat-free tire, primarily for use on industrial or off-road vehicles. The tire-filling urethane elastomer is a soft, resilient, rubbery polymer formed by mixing two reactive liquids with a suitable catalyst, the said mixture then being pumped into the tire through the inflation valve, where it cures to form a solid tire. The catalyst serves to speed up the polymerization reaction, and two types of catalyst are generally used at the present time; one being the amine type, and the other an organo-metallic type. The organo-metallic catalysts are favored over the amine type because they produce a more rapid polymerization reaction and require a minimum of heat to cure the urethane. However, the organo-metallic catalysts frequently have a detrimental effect on the heat stability of the urethane elastomer, and the most effective organo-metallic catalysts usually have the worst effect. For that reason, they are rarely used when high heat-resistance properties are required. The detrimental effect of the organo-metallic catalyst is that it causes the product to revert, or depolymerize at temperatures above 200.degree. F. With depolymerization, the solid urethane elastomer becomes liquid again, and the reaction does not reverse itself when the material cools. As a result, polyurethane elastomer as a tire-filling material has certain limitations, which sometimes restrict its use.
We have discovered that when urethane elastomer catalyzed with organo-metallic catalyst is allowed to cure in contact with rubber, the material immediately adjacent the rubber was considerably harder than the same material at a location more remote from the rubber surface after being subjected to temperatures in excess of 200.degree. F. At the same time, the material adjacent the rubber was discolored by being blackened. The higher hardness of the material immediately adjacent the rubber was a very desirable property, as it indicated a greater resistance to high temperatures.
At first, the discoloration was thought to be due to carbonization of the urethane at the high temperature to which it was exposed, but subsequent investigation led to the conclusion that the blackening effect was caused by reaction of something in the elastomer with something that had migrated from the rubber. Checking all of the known ingredients in ordinary tire rubber, it finally became evident that the ingredient that was causing this heat stabilization effect was sulfur, and what was happening was that the sulfur was migrating from the tire rubber into the elastomer, where it reacted with the catalyst, forming a metallic sulfide with its characteristic black color. With the metallic ions thus removed from the organo-metallic catalyst, the catalyst is completely neutralized, or deactivated, and depolymerization of the urethane at temperatures above 200.degree. F is slowed as the catalyst promotes the depolymerization reaction. A very important factor in this reaction is that the initial polymerization reactions occur at a temperature well below 200.degree. F, at which temperature the sulfur remains inert and does not react with the metallic ions in the catalyst. Thus, the catalyst is able to do its job of accelerating the polymerization of the urethane without interference by the sulfur, and the sulfur then remains inert in the urethane until such time as the temperature rises into the range of 200.degree.-250.degree. F. At this elevated temperature, the sulfur quickly reacts with the catalyst, forming a metallic sulfide, and completely neutralizing the catalyst so that it cannot depolymerize the urethane. The sulfur does not appear to react in any way with the urethane itself, but only with the catalyst, which represents an extremely small percentage of the total.